1. Field of the Invention
The present invention relates to a joined body, a method for manufacturing the same and particularly relates to a high pressure discharge lamp.
2. Description of the Related Art
A high pressure discharge lamp has a ceramic discharge tube with two end portions, in which sealing members (usually referred to as a ceramic plug) are inserted, respectively, to seal the respective end portions. A through hole is formed in each sealing member and a metal member, to which a predetermined electrode system is attached, is inserted within the through hole. An ionizable light-emitting material is introduced and sealed within the inner space of the discharge tube. A known high pressure discharge lamp includes a high pressure sodium vapor and metal halide lamps, the latter exhibiting more superior color coordination. The lamp may be used in high temperature conditions by forming the discharge tube by a ceramic material.
In such discharge lamps, it is necessary to provide an air-tight seal between the end portion of the ceramic discharge tube and a member for supporting an electrode system. The ceramic discharge tube has a main body with a shape of a tube with two narrow ends, or a barrel, or a straight tube. The ceramic discharge tube is made of, for example, an alumina sintered body. The end portion of the ceramic discharge tube may be sealed, for example, by the following method disclosed in Japanese patent publication Kokai (Tokkaihei) 318,435/1994. A sealing member is inserted and supported in the end portion of the ceramic discharge tube. A through hole is formed in the longitudinal direction of the center axis of the sealing member. An elongate member for supporting an electrode system is inserted and fixed within the through hole. The sealing member is made of a cermet containing both of alumina and a metal constituting the member for supporting an electrode system in a predetermined ratio, so that the sealing member has a thermal coefficient between those of the electrode-system supporting member and ceramic discharge tube.
When producing such a sealing structure, the calcined bodies of the ceramic discharge tube and sealing member are designed so that the end portion of the discharge tube has an inner diameter smaller than that of the outer diameter of the sealing member, when the calcined bodies are sintered without inserting the calcined body of the sealing member within the end portion of the calcined body of the tube. Therefore, the sealing member is tightly clamped and held in the end portion of the ceramic discharge tube. The sealing member and the electrode system supporting member are similarly designed.